Spectroscopic Observations (Massey & Hanson 2011, arXiv v2.pdf) Examples of Spectrographs Spectroscopy with CCDs Data Reduction and Calibration Software Resources 1

Spectrograph Examples: LHIRES III on HLCO 20-inch Classical Czerny-Turner made by Shelyak 2400, 600, 150 groves per mm gratings maximum R = two slit plates used with SBIG CCD camera for data and WATEC IZON webcam for acquisition/guiding For details see Jenkins 2011 M.S. thesis: 2

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4 Telescope focus Guider CCD Micrometer to set the grating angle

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MTT Spectrograph at HLCO: Ebert-Fastie design Fiber fed slit composed of 9 vertical fibers Single mirror for collimator/camera Four gratings: 2160 (Lowell), 1200, 600, 50 for R= 40000, 19000, 9000, and 500 (m=1) Variety of order sorting filters Ph.D. dissertations of Don Barry (1996) and Reed Riddle (2000) 6

7 Barry et al. 2002, PASP, 114, 198

Prism spectrogaphs dβ/dλ proportional to 1/λ 3 Objective prism of trailed stars, record low resolution grism = prism with grating on one face grens = lens plus transmission grating 8 pectra/ _spectra.shtml

Fourier Spectroscopy Michelson interferometer (interference from path length difference) recorded intensity is FT of spectrum useful in IR; no slit so very efficient 9

Fabry Perot Interferometer produces interference rings (one per order) result is convolution of image and rings very spacing of etalons or refractive index of gas between lenses to scan image in wavelength (ex. velocity mapping of nebula in a single emission line) 10

Multi-slit spectrographs multiple spectra in a single exposure OK if FOV is small, number of targets is large build metallic mask for focal plane with small “slitlets” at target locations and circular apertures at positions of guide stars central wavelengths shifted by spatial position ex. GMOS/Gemini, IMACS/Magellan 11

Multi-object fiber-fed spectrographs again multiple spectra in a single exposure OK with larger FOV, smaller number of targets micropositioner movers fiber input positions to positions of targets in focal plane (or used with a plug-board grid) output end of fibers stacked in a slit at spectrograph entrance ex. Hectospec/MMT, Hydra/WIYN 12

Integral Field Spectroscopy Obtain spectra at each spatial position in a grid over a small portion of the sky (few arcsec) microlens array fiber bundle combination of above (FLAMES/VLT, GMOS/Gemini) image slicers: sky is sampled by a stack of long, thin mirrors, each of which is rotated with respect to its neighbors (NIFS/Gemini) 13

Spectroscopy with CCD Detectors format, pixel size, spectral DQE read noise, full well potential data frames (“object” with overscan) bias frames (zero exposure) bad pixel mask dark frames flat fields (featureless and twilight) wavelength comparison spectra spectrophotometric flux standard stars 14

Reduction and Calibration Steps fit and subtract overscan region trim dark subtract interpolate over bad pixels construct a master bias frame construct a normalized featureless flat field; sometimes flat division done after spectrum extraction in echelle and multi-object spectra construct illumination correction (long slit) 15

Reduction and Calibration Steps identification of stellar and sky locations across the image spectrum extraction by summation over spatial direction, or better use weighted or “optimal” extraction = spatial profile fit (helps with cosmic ray removal) extract wavelength comparison spectra apply wavelength calibration (heliocentric or barycentric correction) apply flux calibration (or rectify spectra) 16

Software Packages IRAF = Image Reduction and Analysis Facility IDL = Interactive Data Language IDL Astronomy Users Library Python – PySpecKit General tips at AstroBetter Astrophysics Source Code Library 17